Heat pump systems are usually bidirectional, that is, they can provide heating or cooling, or sometimes both simultaneously. It has been known to utilize multiple component CFC or HCFC refrigerants to extend the low temperature end of the useful range for heat pumps when used for heating. However, recent restrictions on the use of CFC's or HCFC's due to the chlorine content that can contribute to the depletion of the earth's ozone layer and global warming, emphasis has now shifted to the use of HFC's and other environmentally acceptable refrigerants. Current electrically-powered residential and small commercial heat pump systems have two operating and performance constraints. Mainly, they have insufficient capacity at low ambient temperatures, below about 30.degree. F. or 40.degree. F., so that supplemental heat sources such as electric resistance heating or fossil fuel fires must be utilized. Additionally, the temperature to which air can be heated by a heat pump working in low ambient temperatures are low for human comfort; air below about 90.degree. F. entering a room provides a draft chill discomfort due to the flow of the air.